After gathering herbs in his garden, Friar Laurence discusses the duality of all things:
O, mickle is the powerful grace that lies
In herbs, plants, stones, and their true qaulities:
For naught so vile that on the earth doth live,
But to the earth some special good doth give;
Nor aught so good, but, strain'd from that fair use,
Revolts from true birth, stumbling on abuse:
Virtue itself turns vice, being misapplied;
And vice sometime by action dignified.
Just as an herb can be used to make medicine or poison, so too can situations have the potential for good or evil, bounty or disaster.
Such duality suffuses every situation and character in the play. There is, of course, the love between Romeo and Juliet itself. Their love is in many ways a good thing: the union between them could force the warring Montagues and Capulets to set aside their differences since they are now literally family, ending the strife in the streets of Verona. Even on a character level, this love is potentially transformative: Juliet grows a more proactive character through her love for Romeo, and Romeo's own poetry improves once he shifts his attentions to Juliet, which is Shakespeare's subtle way of suggesting this love is truer than anything Romeo felt for Rosaline. However, their love also makes both characters prone to rash decisions since they are both still so young and easily swayed by their emotions. Their love, combined with their bad situation and their own immaturity, thus becomes deadly.
Duality is also reflected in the play's themes. Death is horrible and tragic, but the deaths of Romeo and Juliet bring the two families to their senses, ending the feud between them. Love for Juliet makes Romeo willing to turn the other cheek when Tybalt insults him, but when Mercutio is killed, love for his friend throws Romeo into a rage, leading to his killing Tybalt in turn.
Ultimately, Friar Laurence's speech foreshadows the effects of the tragic events to come. Romeo and Juliet's love is both destructive and restorative: while tragic for the individuals involved, for the society of which they are a part, their deaths put an end to the violence plaguing Verona. Both good and evil come of their story.